Reference for a path groupoid being a diffeological groupoid

I am looking for a reference that has a proof that a path groupoid is a groupoid internal to the category of diffeological spaces. I do know how to prove this fact, and a proof is not hard. My reason for asking this question is that I don't want to clutter a paper with things that are apparently well-known. However the nLab article linked to above gives no references.

The places I tried to look this fact up are: Diffeology by Patrick Iglesias-Zemmour, papers of Schreiber and Waldorf, papers of Picken with various collaborators, the nLab, google and google scholar.

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Might be folklore. As a side comment, there's a slightly better way to construct something equivalent, where you don't modify the paths (i.e. use sitting instants) but modify the composition: instead of using the standard diffeo [0,1] -> [0,2], use something that is flat at 1/2. The result seems to me so obvious that one doesn't really need a proof, just a comment. But that doesn't take care of attribution... –  David Roberts Aug 10 '13 at 0:24
David, I also suspect the result is folklore. I just wanted to make sure that it is. –  Eugene Lerman Aug 11 '13 at 16:43
David, I agree that a proof that the concatenation of paths is smooth is easy. The only possible subtlety is that in the groupoid you concatenate thin homotopy classes, and the space of classes has quotient diffeology. So at some point you need to check that in diffeological spaces fiber products commute with quotients, which they do. –  Eugene Lerman Aug 11 '13 at 16:46